The magnetic properties of a transition metal material are widely used by various types of magnetic devices, such as a hard disk drive (HDD), a magnetoresistive random access memory (MRAM), and magnetic sensor. In the technology for manufacturing such magnetic devices, there have been demands for information storages with higher density and sensors with higher accuracy. As a result, processing techniques for miniaturizing magnetic patterns have been diligently developed. In the prior art, as etching techniques used for micro-processing a magnetic thin film, ion milling, which performs physical etching on a magnetic thin film, and reactive ion etching, which performs physical-chemical processing on a magnetic thin film, have been employed.
In ion milling, a magnetic thin film is etched by colliding ions of argon or the like against the magnetic thin film to partially force out portions of the magnetic thin film from its surface. In ion milling, the subject of the etching is the entire region that ions collide against. Thus, etching selectivity is difficult to obtain, and there is a limit to the miniaturization of magnetic patterns. In contrast, in reactive ion etching, the magnetic thin film is placed in a plasma of reactive gas, and the magnetic thin film is removed through physical-chemical reaction between the magnetic material and the reactive gas. This allows for reactive ion etching to have a high etching selectivity in comparison with ion milling.
In the reactive ion etching for magnetic thin films, a gas mixture of O2 and Cl2 or a gas mixture of CO and NH3 is used as the reactive gas. In a micro-processing technique for magnetic thin films, to increase the selection ratio of the magnetic thin film relative to a mask, a mask material having high etching resistance has been proposed for such a reactive gas in the prior art (e.g., patent documents 1 to 4).
In patent documents 1 to 3, a metal layer is formed between a magnetic thin film and a resist pattern, a hard mask is formed by etching the metal layer using the resist pattern, and a magnetic pattern is formed by etching the magnetic thin film using the hard mask. Patent document 1 proposes as the material of a hard mask the use of any one of the materials including Ti, Mg, Al, Ge, Pt, Pd, or an alloy thereof. Patent document 2 uses a metal (e.g., Ta, W, Zr, Hf, etc.) of which melting point or boiling point rises when changed to nitride or carbide. Patent document 3 uses Ti, Al, Ta, W, Co, Mo, Cu, Ni, Fe, and an oxide, nitride, fluoride, boride, and carbide thereof. Patent documents 1 to 3 select the above-described mask material and increase the selection ratio in a CO—NH3 system and an O2—Cl2 system.
In patent document 4, a mask precursor layer and a metal layer are first formed between a magnetic thin film and a resist pattern, a second hard mask is formed by etching the metal layer using the resist pattern, and a first hard mask is formed by etching the mask precursor layer using the second hard mask. A magnetic pattern is formed by etching the magnetic thin film using the first hard mask. Patent document 4 increases the selection ratio between the layers of the resist pattern, the metal layer, and the precursor layer. This increases the selection ratio of the magnetic layer relative to the precursor layer and increases the processing accuracy of the magnetic pattern.
FIGS. 8A to 8D are process charts illustrating a method for manufacturing a magnetic disk used as a perpendicular magnetic recording type HDD. In the magnetic disk manufacturing method, first, a magnetic layer 52, a protective layer 53, a hard mask layer 54, and a resist pattern RM are sequentially stacked from an upper surface side of a substrate 51, as shown in FIG. 8A. For example, when CoCrPt—SiO2 and Ti are respectively selected as the magnetic material and the mask material, the resist pattern RM having a film thickness of 150 nm and the hard mask layer 54 having a film thickness of 20 nm are stacked on the magnetic layer 52 having a film thickness of 20 nm. The protective layer 53 is a thin film layer for protecting the magnetic layer 52 when etching the hard mask layer 54 and is, for example, a diamond like carbon (DLC) layer having a film thickness of several nanometers.
Then, referring to FIG. 8B, reactive ion etching using the resist pattern RM is performed on the hard mask layer 54. Referring to FIG. 8C, a hard mask 54a is formed by ashing the resist pattern RM. Then, referring to FIG. 8D, reactive ion etching using the hard mask 54a is performed on the protective layer 53 and the magnetic layer 52 to form a magnetic pattern 52a. 
In this case, when the magnetic pattern 52a has a design rule (line width and space width) that is less than or equal to 100 nm, the plasma resistance of the resist pattern RM becomes drastically low. This advances deformation of the resist pattern RM, and the hard mask 54a becomes tapered. As a result, the side surface of the magnetic pattern 52a becomes inclined (e.g., inclined 70° or less) relative to the main surface of the substrate 51. As a result, sufficient processing accuracy for performing perpendicular magnetic recording cannot be obtained.
Patent document 4 proposes a hard mask having a stacked structure but does not thoroughly discuss the etching selectivity between the layers of the resist pattern RM, the hard mask layer 54, and the magnetic layer 52. Further, patent document 4 does not thoroughly discuss the mask material for the etching process. Additionally, in patent document 4, when mask layers increase, the number of steps for removing the mask increases. This lowers the productivity of the magnetic device.    Patent Document 1: Japanese Laid-Open Patent Publication NO. 11-175927    Patent Document 2: Japanese Laid-Open Patent Publication NO. 2002-38285    Patent Document 3: Japanese Laid-Open Patent Publication NO. 2002-510142    Patent Document 4: Japanese Laid-Open Patent Publication NO. 2001-229508